Superresolution in optical microscopy and microlithography

ABSTRACT

In scanned optical systems such as confocal laser microscopes wherein a beam of light is focused to a spot in a specimen to excite a fluorescent species or other excitable species in the spot, the effective size of the excitation is made smaller than the size of the spot by providing a beam of light of wavelength adapted to quench the excitation of the excitable species, shaping this second beam into a pattern with a central intensity minimum, and overlapping this central minimum with the central intensity maximum of the focused spot, so that within the spot the intensity of quenching light increases with distance from the center of the spot, thereby preferentially quenching excitation in the peripheral parts of the spot, and thereby reducing the effective size of the excitation and thus improving the resolution of the system. In the preferred embodiment of the present invention, the central minimum of quenching light is narrowed further by creating the pattern of quenching radiation in the specimen by imaging onto the focal plane a plurality of pairs of sources of quenching light, arrayed at the vertices of a regular, even-sided polygon, the center of which is imaged in the specimen on the central maximum of exciting radiation, and such that the two members of each pair are on opposite vertices of the polygon and emit light mutually coherent and out-of-phase, and the light emitted by different pairs is incoherent with respect to each other.

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/919,382, filed on Aug. 28, 1997, now U.S. Pat. No. 5,952,668which is a continuation-in-part of U.S. patent application Ser. No.08/581,185, filed on Dec. 29, 1995 now U.S. Pat. No. 5,777,342, which isa continuation-in-part of U.S. patent application Ser. No. 08/275,967,filed on Jul. 15, 1994 now U.S. Pat. No. 5,866,911.

FIELD OF THE INVENTION

The present invention relates to scanned optical systems in which a beamof light is focused to the smallest possible spot in a specimen in orderto selectively excite, within the illuminated spot, an excitable speciessuch as a fluorescent dye, and more specifically to a method ofimproving the resolution of such systems.

BACKGROUND OF THE INVENTION

In many fields of optics, a light beam is focused to the smallestpossible spot in a specimen in order to selectively photoexcite amolecular species in the illuminated spot. Such fields include scannedbeam fluorescence microscopy, scanned beam microlithography,nanofabrication, and optical digital information storage and retrieval.The lenses in such resolution demanding applications often approachdiffraction limited performance, and in view of the dependence ofresolution on wavelength and numerical aperture of the objectivefocusing the light, these lenses are designed with the largest practicalnumerical apertures and used with light of the shortest practicalwavelengths.

Additionally, a variety of techniques have been devised to pushresolution beyond the Abbe limit set by diffraction theory (S. Inoué, p.85 in D. L. Taylor and Yu-li Wang, Fluorescence Microscopy of LivingCells in Culture, Part B, Academic Press, 1989). These techniquesinclude placing annular and multiannular apertures in the aperture planeof the objective (Toraldo di Francia, Nuovo Cimento, Suppl. 9:426(1952)) and using scanned confocal optics (M. Minsky, U.S. Pat. No.3,013,467 (1961)). While in theory, such aperture plane apertures canallow arbitrarily narrow central maxima of the point spread function,any substantial narrowing of the central maximum is accompanied bydramatically less efficient light utilization and degraded imagecontrast. Although, as originally pointed out by the inventor of thepresent invention (Baer, U.S. Pat. No. 3,705,755 (1972)), this problemof degraded contrast can be reduced by the use of such aperture planeapertures in a confocal scanning system, such a solution does nothing toimprove the inefficient use of light actually reaching the specimen, sothat in practice, light induced damage of the specimen or photobleachingof the fluorescent dye could limit the usefulness of such an approach.The technique of scanned probe, near field microscopy (Lewis et al U.S.Pat. No. 4,917,462) has had more success in achieving high resolution,but this technique is limited to the exposed surface of flat specimens.A related technique applicable only in the special case of optical discrecording and playback, involves the deposition, adjacent to theinformation containing layer, of an opaque layer which is be made toundergo a change an optical property such as transparency by a focusedbeam (Fukumoto and Kubota, Jpn. J. Appl. Phys. 31:529 (1992) Yanagisawaand Ohsawa, Jpn. J. Appl. Phys. 32:1971(1993), Spruit et al, U.S. Pat.No. 5,153,873 (1992)).

Though the variety of proposed superresolution techniques attests to thelong recognized need to improve the resolution of the light microscopefor applications such as the far-field imaging of typical specimens suchas sections of biological tissue, it appears that the practical gainsfor such applications have been effectively limited to less than adoubling of resolving power relative to the Abbe limit. Thus any systemwhich could increase resolution beyond the state of the art, andespecially one which could work in conjunction with presentsuperresolution techniques to further extend resolution performance,could be of great value in the field of light microscopy and otherfields of scanned optics.

OBJECTS AND ADVANTAGES

It is the primary object of the present invention to improve resolutionin optical systems such as scanned fluorescent microscopes, in which, ateach moment, a beam of light is focused to the smallest possible spot ina specimen to excite an excitable species in the spot.

Another object of the present invention, in such systems, is to minimizethe light induced damage to a specimen resulting from photodynamicaction.

Another object of the present invention, in such systems, is to minimizelight induced bleaching and photolysis of the molecules responsible forabsorption and emission.

Another object is to produce a method of fluorescent microscoperesolution enhancement which is easily adapted to current laser confocalmicroscopes, two-photon excitation laser scanning microscopes, andfluorescent decay time contrast microscopes.

Another object is provide a method of resolution enhancement which willwork synergistically with known superresolution methods therebyincreasing the resolution over these known techniques.

Another object is to allow high resolution epiillumination imaging ofliving biological specimens at greater tissue depths from the surfacethan is possible with current techniques.

Another object is to provide a resolution enhancement technology whichcan be adapted to the fields of high resolution photolithography,nanofabrication and digital computer memory storage and retrieval.

Another object of the invention is to avoid image degradation due tocoherence effects of laser illumination, while using such coherenceinstead to increase resolution.

Still other advantages of the present invention will become evident inthis disclosure.

SUMMARY OF THE INVENTION

The foregoing objects are achieved and the foregoing problems are solvedin one illustrative embodiment of the invention, applied specifically tothe field of fluorescence microscopy, although the principles embodiedtherein also apply to the other applications of the present inventiondiscussed in this specification. This embodiment is an improvement ofthe laser scanning fluorescence microscope, wherein a scanned excitationbeam is focused to a diffraction limited spot size and illuminatessuccessive spots in a fluorescent specimen, exciting fluorescent dyemolecules within these spots to fluorescence. Fluorescent lightemanating from each of these illuminated spots is then electronicallymeasured, and a spot of light the intensity of which varies inaccordance with the measured fluorescence from these illuminated spotsis scanned over a video monitor screen in correspondence with thescanning of the excitation beam over the specimen, to create a finalimage of the specimen. In the present invention, light of a wavelengthadapted to quench fluorescent excitation of the excited dye molecules isfocused in the specimen to a pattern containing a central minimum whichis made concentric with the central maximum of the exciting radiation,the central points of the central maximum of the exciting beam and ofthe central minimum of the quenching beam substantially coinciding, sothat within the central minimum region, the intensity of the quenchingbeam, and consequently the degree of quenching of the fluorescence,increases with distance from the central point, thereby decreasing theeffective width of the distribution of probability of fluorescentexcitation as a function of distance from the center of the illuminatedspot, and consequently increasing the effective resolving power of themicroscope. In the preferred embodiment of the present invention, thecentral minimum is narrowed by creating the pattern of quenchingradiation in the specimen by imaging onto the focal plane a plurality ofpairs of sources of quenching light, arrayed in a regular, even-sidedpolygon, such that the two members of each pair are on opposite verticesof the polygon and emit light mutually coherent and out-of-phase, andthe light emitted by different pairs is incoherent with respect to eachother.

BRIEF DESCRIPTION OF THE DRAWINGS

The principles of the invention will be more particularly discussed withreference to the accompanying drawings in which:

FIG. 1 is a schematic cross-sectional view of a scanning fluorescencemicroscope embodying the invention;

FIG. 2 is an enlarged detail showing a portion of the device shown inFIG. 1;

FIG. 3 is a perspective view showing a portion of the device of FIG. 1;

FIG. 4 is a graph showing the intensity distributions at the plane offocus for the excitation and quenching beams, and the resultantfluorescence excitation probability distribution due to the jointinteraction of these beams with the specimen;

FIG. 5 is a schematic cross-sectional view of a portion of a variant ofthe device shown in FIG. 1 to illustrate the use of an annular aperturein an aperture plane to further increase resolving power;

FIG. 6 is a schematic cross-sectional view of an embodiment of thepresent invention using slit apertures;

FIG. 7 is a schematic cross-sectional view of the present inventionwherein the excitation and quenching beams are presented as successiveultrashort pulses;

FIG. 8 is a schematic cross-sectional view of the present inventionwherein the resolution of the device is improved by forming thequenching beam by imaging onto the specimen the ends of a set oflaser-illuminated optical fibers arrayed at the vertices of an evensided polygon, such that the laser output of diagonally opposite fibersis out-of-phase, and the laser output of adjacent fibers is mutuallyincoherent;

FIG. 9 is a view showing the arrangement of the non-illuminated ends ofthe fibers in the device shown in FIG. 8;

FIG. 10 is a schematic cross-sectional view of an embodiment of thepresent invention combining the features of the devices illustrated inFIGS. 7 and 8;

FIG. 11 is a diagram showing the arrangement of the non-illuminated endsof the fibers in the device of FIG. 10;

FIG. 12 is a schematic cross-sectional view of an embodiment of thepresent invention applied to microlithography;

FIG. 13 is diagram showing the ends of the quenching beam optical fibersin the device of FIG. 12, in the plane conjugate to the photoresistlayer;

FIG. 14 is a diagram showing the direction of translation of the mask inthe device of FIG. 12.

FIG. 15 is a perspective view showing a detail from an alternativeembodiment to the device of FIG. 1, where the pattern of quenchingradiation is produced by passing the quenching light through a filterhaving regions of differing phase shifts.

FIG. 16 is a enlarged view of the filter in the device of FIG. 15,showing the phase shifts for various regions.

FIG. 17 is view of a filter having regions of different phase shiftsarranged in a square matrix.

FIG. 18 is view showing a filter showing regions with eight distinctphase shifts, arrayed in a square matrix.

FIG. 19 is a schematic cross sectional view showing a microlithographicembodiment of the present invention, where the pattern of quenchingradiation is produced by a matrix similar to that shown in FIG. 17.

FIG. 20 is a schematic cross sectional view showing how a dispersioncompensating assembly can allow production of sources of quenchingradiation where all the wavelength components have the same 180° phaseshift.

FIG. 21 is a schematic cross sectional view showing a form of microscopeembodiment of the present invention, where many points aresimultaneously viewed.

DESCRIPTION OF THE INVENTION

FIG. 1 shows an embodiment of the present invention employing continuouswave laser illumination. Light from excitation laser 10 is focused bylens 12 onto pinhole aperture 13, and after passing through aperture 13,reflection from dichroic mirror beam splitters 14 and 15 and scanning bybeam scanning means 16 (which may be a pair of orthogonal galvanometerpowered scanning mirrors) the laser light is imaged by eyepiece 17 andobjective 18 on or within the specimen 19 stained with fluorescentmolecules excitable by light emitted by laser 10, to form a region ofexcited fluorescent molecules at the image of pinhole aperture 13.Quenching laser 11, which emits light of a wavelength adapted to quench,by means of stimulated emission, the fluorescent excitation caused bylaser 10, is focused by toroidal lens 22 onto annular aperture 21. Lightpassing through aperture 21 passes through dichroic beam splitter 14,then is reflected by dichroic beam splitter 15 to pass through the beamscanning means 16, and is focused by eyepiece 17 and objective 18 ontospecimen 19. In FIG. 1, for purposes of illustration only, rays are showemanating from the central point of annular aperture 21 to simplify theillustration and to show that this central point is conjugate, withrespect to beam splitter 14, to the central point of pinhole aperture13, and it should be understood that since this central region ofannular aperture 21 is in fact opaque, rays do not actually emanate fromthis central point. In FIG. 2, a magnified detail showing laser 11,toroidal lens 22 and annular aperture 21, the rays are correctly shownfocused by lens 22 onto and then emanating from the transparent ring ofaperture 21. The mirror image relationship between the center of annularaperture 21 and pinhole aperture 13 insures that the projected image inthe specimen 19 of annular aperture 21 is concentric with the image inthe specimen of pinhole aperture 13. The diameter of annular aperture 21is chosen so that the diameter in the specimen of the central ring ofmaximum intensity is the same as the diameter of the first minimum of anAiry disc point diffraction image which would be formed at thewavelength of the quenching laser 11. This means that the diameter inthe specimen of the ring of maximum intensity for the quenchingradiation is larger than the diameter of the first minimum of the pointdiffraction image of pinhole 13 by the ratio of the wavelength of thequenching laser to the wavelength of the excitation laser.

Objective 18 collects fluorescent emission from those excitedfluorescent molecules in the specimen, in the focus of the excitationbeam, which have not been quenched by the quenching beam. This emission,after passing through objective 18 is directed successively througheyepiece 17, beam scanning means 16, beam splitter 15, and blockingfilter 27 adapted to block reflected excitation and quenching light, toviewing pinhole aperture 22, which is conjugate to pinhole aperture 13.Emitted light passing through aperture 22 is detected by photodetector23 (which may be a photomultiplier tube) the output of which is directedto video frame store 24, which is synchronized by the scan drive circuit25 which powers the beam scanning means 16. The information contents ofthe video frame store 24, as manipulated by appropriate image processingmeans, is displayed on video monitor 26, producing an image of thescanned plane of the specimen 19.

FIG. 3 shows a perspective detail of a portion of the apparatus of FIG.1. It should be noted that for purposes of illustration, the openings ofpinhole aperture 13 and annular aperture 21 are shown larger than thescale of the rest of the elements in this figure. Although in principle,completely different optical systems could be used to project a centralminimum of quenching radiation so its center coincides in the specimenwith the central point of the central maximum of the exciting radiation,for use in a scanning microscope application, the sharing of focusingoptics, made possible by the use of beam splitter 14, insures thatprovided the focusing and scanning systems are achromatic, the requiredcoincidence between the central points of these images will beguaranteed even at the precision required in high resolution microscopy,once the apertures 13 and 21 are aligned at one scan position, becauseperturbations, for example due to inhomogeneities in the specimen abovethe plane of focus, distort the excitation and quenching beams equally.In case focusing optics which have been achromatized for the choice ofexcitation and quenching wavelengths are unavailable, it is possible tolongitudinally shift aperture 21 relative to the position conjugate toaperture 13 with respect to beam splitter 14, so that the images of thetwo apertures are coplanar and concentric in the specimen. Since such anarrangement will only provide correction for the axial focal point,scanning can be provided in such an arrangement, for example by lateralmovement of the specimen.

Why the present invention will cause an improvement in resolving poweris shown in FIG. 4, which shows how quenching, applied according thepresent invention, can reduce the nominal width of the distributionindicating the probability that a given probe fluorescent molecule inthe specimen will be in its excited state, as a function of the distanceof this molecule from the center of the central maximum of the focusedexcitation beam. To simplify this illustration, it is assumed that thisprobability of excitation of the fluorescent molecule is proportional tothe intensity of exciting illumination incident on the molecule.Secondly, the illustration is applied specifically to the imaging of thecentral spot of the imaging field, where the center of the spotintersects the optical axis of the objective 18.

FIG. 4A shows the expected light intensity distribution of the “Airydisc” or diffraction image of excitation pinhole aperture 13 projectedin the specimen 19 by objective 18. The use of the axial point as theexample in this illustration means that the spatial coordinate in thedistribution is simply the distance, in the plane of focus, from theaxis of objective 17, this distance being shown by distance from thevertical line 31 common to FIGS. 4A, B, C and D. The vertical axis inthese figures represents either light intensity or excitationprobability, where the upward direction corresponds to increases. Thedescription of imaging of non-axial points is somewhat more complicatedthan this present axial case, but resolution enhancement works in thesame way. FIG. 4B shows the probability of excitation 30 of afluorescent molecule as a function of the distance in the focal planebetween the axis and that molecule. The assumption of a proportionalexcitation response by the fluorescent molecule means that thisexcitation probability distribution 30 shown in FIG. 4B is proportionalto the diffraction image distribution shown in FIG. 4A. In a case wherethe excitation probability was not proportional to excitation intensity,for example because excitation saturated the population of fluorescentmolecules causing the top of the central maximum of curve 30 in FIG. 4Bto be flattened with respect to the curve of FIG. 4A, the followingarguments for the shrinkage of the width of the excitation probabilitycurve are still valid.

The nominal width of the central maximum of the excitation probabilitydistribution (without any resolution improvement due to quenching) isshown by the double arrow 32, which measures the distance between thetwo points in the distribution where the probability of excitation ishalf maximal. (When this distribution is imagined in the two dimensionsof the plane of focus, double arrow 32 indicates the diameter of thecircle where the probability of excitation is half the maximumprobability). The object of the present invention is to apply thequenching radiation in a pattern which preferentially decreases theprobability of resulting excitation in peripheral portions of thecentral maximum of the excitation probability curve, while sparing, asmuch as possible, the probability of excitation in the central portion,thereby narrowing the nominal width of the probability of excitationcurve.

FIG. 4C shows the expected intensity distribution 33 of the image,projected in the specimen, of annular aperture 21, which is illuminatedby the quenching beam. The mean radius of annular aperture 21 is suchthat, by diffraction theory, the diffraction image in the specimenresulting from the contribution from of each small section on the ringof annular aperture 21, taken in isolation, has an intensity of zero onthe optical axis (i.e., the first minimum passes through the opticalaxis), so that as the light emanating from each of the small sections onthe ring of aperture 21 summates to create an image of annular aperture21 in the specimen, the sum, at this central axial point, of the zerointensities from each of the sections of the aperture 21 still adds tozero. However, scattering in the optics of the instrument and in thespecimen, and reflections from lower lying layers of the specimen,causes the central minimum on the optical axis, in fact, to have a smallbut finite intensity I_(b) shown by the horizontal line 35, which isabove the zero intensity baseline 36. The term “central minimum” is usedherein analogously to the more common expression “central maximum” andrefers to the fact the image has a minimum at its center, even if theimage as a whole is not centered with respect to the optical axis, aswhen non-axial points are imaged.

In the following discussion, it is assumed that once a given fluorescentmolecule is excited, it has a probability p of eventually emitting afluorescent photon, and that for a wide range of initial conditions,such as different mixtures of exciting vs. quenching light, anddifferent concentrations of fluorescent molecules, that adding to theexisting light mixture the same intensity of quenching light, called150, which is different for different species of fluorescent molecule,will reduce p to half its value before such addition. It should beemphasized that the assumption may not be completely valid in view offactors such as saturation, but it will nevertheless help illustrateseveral aspects of how quenching can improve resolution.

For a given total power of the quenching beam and a given species offluorescent molecule, it is possible to see approximately how muchsharpening of resolution will result with the present system bydetermining the intensity of quenching radiation which must be mixedwith an excitation beam to reduce its effective rate of excitation ofthe fluorescent molecules by 50%. This intensity is shown by the doublearrow labeled I₅₀ in FIG. 4C′, which shows a detail from FIG. 4C inmagnification.

It is assumed that due to factors such as scattering in the optics andthe specimen, there is a small but finite intensity of the quenchingbeam at the central point of the central minimum of the quenching beam,shown by the double arrow labeled I_(b). The addition of more quenchinglight of intensity I₅₀ therefore will bring the total intensity ofquenching light to the level shown by the line 37 which has a intensityof I_(b)+I₅₀. By definition of I₅₀, then, at the distance from thecentral maximum where the intensity of the quenching beam is I_(b)+I₅₀,shown by the vertical lines 39 and 40, the efficiency of excitation inproducing a latent image is half the efficiency in the center of thecentral maximum. Because the excitation beam is also most intense at thecenter, the full width at half maximum of theprobability-of-fluorescent-emission curve, post quenching, is actuallynarrower than the distance between lines 39 and 40, shown by the doublearrow 41. It can be seen that by simply increasing the total power ofthe quenching beam, that the double arrow 41 can be arbitrarily reduced.FIG. 4D shows the distribution 38 of the probability of fluorescentmolecule excitation, subject to quenching by the quenching beam (theeffective excitation, correlated with the probability of ultimatefluorescence emission), and it can be understood that this distributioncan be arbitrarily narrowed, by reducing double arrow 41 by means ofincreasing the quenching beam power.

Of course, there is a limit to how much quenching radiation can bedirected onto the specimen before it is damaged by heating, thereforethe ability of the specimen to tolerate high quenching beam powers maybe the major resolution determining factor in the present system formicroscopy or microchip fabrication. Furthermore, it will be appreciatedthat as the total power for the quenching beam increases, so does theintensity of the central point of the central minimum, I_(b), andconsequently the effective sensitivity of the process is reduced.Therefore another design objective in the present system is to reducethe intensity of the central point of the central minimum of thequenching beam to the lowest practical level, in order to preserve thesensitivity of the process, while achieving good resolution improvement.

FIG. 4 also shows that only a small part of the energy of the quenchingbeam, in the central part where the intensity is lowest, is involved inresolution enhancement. This means that for each milliwatt of laserenergy needed in the crucial central part of the quenching laser beam, atotal beam power of perhaps hundreds of milliwatts may be required.However such powers are easily attainable with available lasers.Furthermore, high intensities of the quenching beam in the bright ringsurrounding the central minimum do not degrade the image because thefinal excitation probability of the fluorescent molecule can never belower than zero, so high quenching intensities will saturate at zero netexcitation. Therefore, from the point of view of image quality, theintensity of the quenching beam can be adjusted for optimal sharpeningat the center of the intensity minimum, without worry about the highintensities surrounding the central minimum. By choice of a quenchingwavelength where there is negligible absorption by the specimen exceptby excited fluorescent molecules, thermal effects on the specimen of thequenching beam are minimized. Thermal effects might also be reduced byuse of diamond or other high thermal conductivity material as a supportfor the specimen.

Another possible concern about the high intensity of quenching radiationis that some stray radiation could enter the photodetector, degradingimage contrast. However both filter 27 and dichroic mirror 15 block suchquenching radiation from entering the detector. Additionally, in formsof the invention using pulsed radiation, quenching light can beeliminated by gating off the detector sensitivity during times thequenching light is on. From the point of view of specimen andfluorophore damage, in the presence of excited fluorophores capable ofproducing photodynamic damage, quenching radiation can reduce suchdamage by deexciting the excited species. Therefore, by choice of aquenching wavelength where there is negligible absorption by thespecimen except by excited fluorophore molecules, high quenchingintensities are not simply tolerable, but can actually be beneficial.

Under same the conditions described above to produce the minimum in thefocal plane, which is perpendicular to the optical axis, the intensitydistribution measured along the optical axis also has a minimum sharingthe same central point. With microscope objectives generally, the widthof the central maximum of the point diffraction image, measured in thefocal plane perpendicular to the optical axis, is smaller than the widthmeasured along the optical axis resulting in a better lateral resolutionthan longitudinal resolution. This same elongation along the opticalaxis occurs with the central minimum of the diffraction image of theannular aperture 21, so that in general, following resolutionenhancement by the apparatus shown in FIG. 1, longitudinal resolutionwill also be improved by quench sharpening, but the lateral resolutionwill still be better than the longitudinal resolution.

Because (at least near the focal point) excitation of the fluorescentmolecule due to the cone of rays converging to the focal point and thecone of rays diverging from the focal point is eliminated by quenching,only the in-focus rays at the focal point remain for production of thefluorescent image. Therefore the present system (in common with confocalmicroscopes) permits extending the practical depth-of-focus, by scanningin depth in addition to scanning laterally. This is particularly usefulin more parallel forms of image formation discussed below, where thesame image would be formed at many different closely spaced focussettings, to produce a composite image which is sharp at every depth ofthe specimen. Additionally, by increasing the intensity of the image fordeeper layers, it is possible to compensate for absorption of theexcitation light by superficial layers of the specimen.

FIG. 5 shows how the resolution of the present invention might beincreased by combining the field plane annular aperture used in thepresent invention to create in the specimen a quenching beam patternwith a central minimum, with an aperture plane annular aperture toimprove the lateral resolution of that quenching beam pattern. (Born andWhite, Principles of Optics, 3rd edition, p. 416). A relay lens systemconsisting of lenses 45 and 46 with annular aperture 47 in an apertureplane, images annular aperture 21 onto a real image plane 48 which isconjugate to pinhole 13 relative to the beam splitting mirror 14 (i.e.,real image plane 48 is in the same plane as annular aperture 21 in FIG.1 before being displaced to the position shown in FIG. 5). The diameterof annular aperture 21 is readjusted to cause the central minimum in itsimage in the specimen to have a minimum intensity. The use of anaperture plane annular aperture 47 in addition to the field planeannular aperture 21, causes the central minimum in the specimen to havea smaller width, to improve lateral resolution, and at the same time tohave a reduced resolution in the axial dimension for improveddepth-of-field. Instead of the annular aperture 47 in the aperture planeof the relay lens system, there can be a complex aperture of differentannuli, each with specified phase retardation and opacity, for furtherdecreases in the width of the central maximum, as shown originally byToraldo di Francia (Nuovo Cimento, Suppl. 9:426 (1952)). It should benoted that in the design of such an aperture plane aperture 47, it ismore important to maximize the sharpness of the first minimum of thepoint spread function rather than the usual design criterion ofmaximizing the sharpness of the central maximum. (The central minimummight also be “sculpted” to optimize, for example, a particular desiredtradeoff between image brightness and resolution, by replacing theuniform ring of aperture 21 with a series of annuli of independentlycontrollable phase retardation and absorption, in addition to the choiceof phase retardations and absorptions for annular rings in the apertureplane aperture 47.) It should also be appreciated that a major problemwith Toraldo type aperture plane apertures, namely the defection of beampower away from the central maximum, causing problems of reducedcontrast and increased photobleaching and photodynamic specimen damage,are overcome in the present invention, because, as described above,light from the quenching beam outside the central minimum, does notdegrade image contrast or increase photobleaching or photodynamicdamage.

FIG. 6 shows an embodiment of the present invention where the pinholeaperture 13 of FIG. 1 is replaced by a slit aperture 60, which isilluminated by the same excitation laser 10 as in FIG. 1, but where thebeam emanating therefrom is focused by cylindrical lens 61 to a linecoincident with the slit in aperture 60. Slit aperture 60 in thecross-sectional view of FIG. 6 appears identical to the cross-sectionalview of pinhole aperture 13 in FIG. 1. The beam emanating from aperture60 is reflected successively by dichroic mirror beam splitters 14 and15, identical to those shown in the device of FIG. 1, and is passedthrough beam scanning means 62, which differs from the scanning means 16of FIG. 1 because it is required to scan in just one dimension. Thescanned beam is focused successively by the eyepiece 17 and objective 18lenses onto specimen 19, identical to those of FIG. 1, however the imageof the excitation beam in the specimen 19 is an illuminated strip, witha central maximum which is elongate in the dimension parallel to thestrip. The nominal width of such an elongate central maximum is definedherein as the distance between the lines where the intensity ishalf-maximal. The quenching laser 11 is directed on two parallel slits63 and 64, by means of two parallel cylindrical lenses 65 and 66. Thespacing between slits 63 and 64 is such that the first minima of theirdiffraction images in the specimen 19 coincide to produce a centralminimum, made to coincide with the central maximum of the diffractionimage in the specimen of slit 60, such that the central line of thecentral maximum coincides with the central line of the central minimumof the focused quenching light. Fluorescent emission from specimen 19 isfocused by successive objective 18 and eyepiece 17 lenses to focus lightfrom the central maximum in the specimen onto a linear photodiode array67, oriented perpendicular to the plane of FIG. 6. The output from array67 is stored in video frame store 24 which is synchronized by the outputof the scan drive circuit 68 which drives the one dimensional beamscanner 62.

The advantage of a one-dimensionally scanned strip arrangement as inFIG. 6 compared with a two dimensionally scanned spot arrangement as inFIG. 1, is that one less dimension in scanning is required, so muchfaster scans at a higher scan frequency can be produced, and theapparatus is simpler. The disadvantage is that the resolution gain ofthe present invention is secured only in one dimension. However for manyapplications a gain in resolution in just one dimension is sufficient,and the simplicity and scanning speed of the slit arrangement arepreferred. It should be noted that instead scanning produced by beamscanning means 16 or 62, the required relative movement between specimenand the image of the focused light beams in the specimen can be producedby movement of the specimen, or an optical element in the light pathbetween the light source and the specimen, synchronized with the imageacquisition process of frame store 24.

FIG. 7 illustrates an embodiment of the present invention employingsynchronized ultrashort-pulse (shorter than a few picoseconds),repetitively pulsing lasers for excitation and quenching. In particular,the pulse output from the lasers is adjusted so that the quenching beamis turned on within picoseconds of the offset of the excitation beam,before there has been any time for significant fluorescent emission, sothat virtually all such emission will follow the offset of the quenchingbeam. There are significant advantages of such a pulsed laser embodimentof the present invention compared to embodiments wherein the excitationand quenching beams are continuously on. Most importantly, there is moreefficient quenching per watt of average quenching beam power. This maybe understood from a specific example where the excitation pulsefrequency is assumed to be 100 MHz, and the fluorophore is assumed tohave a 1 ns half-life for the excited state. (For the purpose of thisexample it is assumed that the fluorophore is very efficient, so in theabsence of optical quenching, the excited state decays almostexclusively by fluorescent emission.) If it is assumed that eachquenching photon incident on the fluorophore has a 20% probability ofquenching it, then 10 incident photons together would have about a 90%chance of producing quenching. If these 10 quenching photons weredelivered within several picoseconds following each excitation, thensubstantially all the quenching would take place before there was anopportunity for fluorescence so the resulting quenching would be 90%.However if these 10 quenching photons were emitted by a continuous wavelaser, so they arrived spaced over the 10 ns interval between excitationpulses, after the first nanosecond, there would have been roughly a 50%likelihood of fluorescent emission, but only roughly a 20% likelihood ofquenching, and obviously any quenching photons which arrive after thefluorescent emission have zero effect. In other words, bunching thephotons in the interval immediately after excitation greatly improvesthe quenching efficiency. The quenching efficiency can be furtherincreased in a pulsed system by making the excitation and quenchinglasers have the same polarization, so there is insufficient time for asignificant change in direction of polarization by rotation of thefluorophore between excitation and quenching, hence the quenching laserwill be optimally aligned with the excited molecules.

The pulsed laser embodiment of the present invention shown in FIG. 7 hasadditional advantages. The ultrafast laser excitation makes itconvenient to excite fluorescence by two-photon absorption (Denk, et alScience 248:73 (1990), Denk, et al U.S. Pat. No. 5,034,613 (1991)),which substantially confines light induced damage and photobleaching tothe plane of focus, and provides illumination with light of a relativelylong wavelength which can penetrate to greater depths of tissue.Unfortunately, with state of the art two-photon microscopy, theadvantage of limitation of excitation to the plane of focus is gained(for a given fluorophore) only with some loss of lateral resolution(Sheppard and Gu, Optik 86:104 (1990)). However in the presentinvention, the quenching beam rather than the excitation beam is theprincipal determiner of lateral resolution, so that two-photonexcitation can be used for excitation, with the resulting confinement inexcitation, and the use of single-photon absorption for quenchinginsures high lateral resolution. Still an additional advantage of thepulsed embodiment shown in FIG. 7 is that it allows laser dyes and theirlocal environments to be characterized by fluorescence lifetimemeasurements, with minimal additional equipment costs. Furthermore, witha photodetector 23 that is not blinded by direct reflections from thelaser pulses, time discrimination can replace the wavelengthdiscriminating blocking filter 27, thus avoiding one source of potentiallight wastage.

In FIG. 7, excitation laser 70 and quenching laser 71 are ultrashortpulse mode-locked lasers, using, for example, optically pumpedTi-sapphire, dye or Cr-forsterite as the active medium. These types oflasers result in a repetitive train of pulses of durations from about100 femtoseconds to several picoseconds, and at a frequency of about 50to 100 MHz which depends on the length of the laser cavity. Lasers 70and 71 are synchronized by means of a phased locked loop synchronizingcircuit 73, which, by means of phase detector 74 detects phasedifference between the amplified and filtered electrical outputs of highfrequency response photodetectors 75 and 76, which receive a portion ofthe output beams of lasers 70 and 71 respectively by means of beamsplitters 77 and 78. The output 79 of circuit 73, representing acorrection signal to stabilize the desired phase difference between thepulse trains from the two lasers, is applied to a piezoelectric actuator80 which controls the longitudinal position of one of the end mirrors 81of the cavity of laser 70, thereby adjusting the laser pulse frequency,and thus stabilizing this phase difference. The desired phasedifference, where the pulse from laser 71 follows the offset of thepulse from laser 70 by an interval from zero to several picoseconds canbe adjusted either electrically in circuit 74, for example by adding acontrolled phase shift to one of the inputs of phase detector 74, oroptically by means of adjusting the optical path difference between theoutputs of the two lasers. A commercially available unit to implementcircuit 73 is the Model 3930 Lok-to-Clock™ Electronics Control fromSpectra-Physics Lasers, Inc., Mountain View, Calif., which can be usedwhen the lasers 70 and 71 are Spectra-Physics Model 3960C Tsunami™Ti-sapphire lasers. FIG. 7 also shows a frequency doubling crystal 82,which can be optionally placed in the beam path of the quenching laserto halve the output wavelength. This doubling crystal 82 isrepresentative of such frequency multiplying means which can be placedin the path of either laser to change the output wavelength. Both lasers70 and 71 may be pumped by a common argon ion laser (unillustrated) witha divided output. Apart from the lasers 70 and 71 and theirsynchronizing apparatus, all the elements in the embodiment shown inFIG. 7 are substantially identical to and serve substantially theidentical function to the respective elements shown in FIG. 1, so theyare numbered with the same numerals as FIG. 1, and are described by thetext corresponding to FIG. 1.

Many alternative methods of producing two beams of synchronizedultrashort pulses are known in the art. The synchronization betweenlasers 70 and 71 could be by means of a purely optical coupling, forexample by having both lasers optically pumped by the pulsed output of asynchronous pumping laser (Moritz, N. et al, Optics Comm.103:461(1993)). Still another possibility is for a portion of the lightoutput of the excitation laser 70 to be used as a synchronous pumpenergy source for the quenching laser 71 (or vice versa). Anotherpossibility is to provide a single laser which emits ultrashort pulsesin the near i.r., for example the 1.3 μm output of a Cr-forsterite laserin a self-modelock configuration, or the 1.5 μm, 2 ps, 27 MHz output ofa Er, Yb doped fiber laser (Laser Focus World, July 1993 p. 15) andsplit the output into a portion directed through a frequency doublercrystal and a frequency tripler crystal to derive the quenching andexcitation beams respectively. Furthermore, the recently commerciallyavailable optical parametric oscillator coherent light sourcesintrinsically produce simultaneous outputs at several wavelengths,which, if necessary, can be frequency multiplied to the range requiredfor a broad range of fluorescent dyes. Finally it has been possible toproduce two color pulses from the same laser (de Barros M. R. X. andPecker, P. C., Optics Lett. 18:631(1993)), and the outputs could beseparated by wavelength for use as the excitation and quenching beams.

The quenching radiation emitted by laser 11 or 71 must be of awavelength adapted to induce stimulated emission from the fluorescentdye molecules in specimen 19, and consequently must be of a wavelengthwhere there is significant fluorescent emission. Furthermore, it isimportant that this quenching radiation not itself fluorescently exciteground state fluorescent molecules. These simultaneous requirements maybe met in several ways. In dyes such as the coumerin derivatives withrelatively large Stokes shifts, the excitation spectrum has dropped to anegligible level in the long wavelength portion of the fluorescentemission spectrum. Alternatively, in a fluorescent dye with a largeprobability for transition between the ground vibrational level of thefirst electronically excited singlet (i. e., the fluorescently excitedstate) to the second vibrational level of the ground state, such that atthe wavelength corresponding to this transition, there is still asignificant emission likelihood, and hence a significant stimulatedemission cross-section, but the wavelength is long enough that theabsorption of the quenching light by the ground state fluorophores hasdropped to essentially zero. When used for biological microscopy,additional desired attributes for a dye in the present application whichare also generally desirable for any fluorescent dye in a biologicalmicroscopy application are that it have a high fluorescent efficiency,that it be commercially available in forms such as antibody and dextranconjugates, and that it have a low intersystem crossing probability forproduction of triplet states (or be self quenching for tripletexcitation). A large two-photon excitation cross-section leaves open thepossibility of excitation by two-photon excitation. Finally, theexcitation and quenching wavelengths must be chosen with respect to costand availability limitations of the excitation and quenching lasers.

The choice of wavelength of excitation and quenching is also subject toa tradeoff since shorter wavelengths lead to increased resolution by theclassical resolution criteria, whereas longer wavelengths, especiallyabove 630 nm (Puppels, G. J., et al, Exptl. Cell. Res. 195:361(1991))are reported to be less toxic to biological tissue at high powerdensities and can penetrate biological tissue with less scattering. Infact, as discussed, the lowering of scattering may be more critical toachieving good resolution in the present invention than the resolutionperformance as predicted by diffraction theory for a non-scatteringmedium, because it may allow a lower intensity at the central minimum ofthe quenching beam focus. In case it is necessary to use a quenchingbeam in a part of the spectrum which can be injurious to the specimen,these quenching photons can be used most efficiently by insuring thatthe just preceding excitation pulse was of sufficient intensity tonearly saturate the fluorescent excitation. For relatively inefficientfluorescent dyes this may require reducing the frequency of the pulseoutput of laser 70, so that for a given time averaged power output, thepower per pulse increases. On the other hand, with an efficientfluorophore, there might be near saturation with each pulse, even with afew milliwatts average beam power and a frequency of about 80 MHz. (seeTsien and Waggoner, Fluorophores for Confocal Microscopy, in Handbook ofConfocal Microscopy, James B. Pawley, ed., Plenum, New York, 1990).

The recently developed cyanine dye, Cy5, has been reported to be easilyconjugated to antibodies, avidin, DNA and other molecules important influorescence biomicroscopy and, in addition, possesses the desirablequalities of a high quantum efficiency, stability and long wavelengthexcitation (Majumdar, et al, Bioconjugate Chem. 4:105 (1993)). Theembodiment illustrated in FIG. 7, when outfitted with Ti-sapphire laserscould excite Cy5 by setting the excitation laser 70 at the 680 nm lowwavelength end of the laser's tuning range, and quench by setting laser71 at about 740 nm. The coumarin dyes are commercially availableconjugated to molecules useful in fluorescence microscopy (MolecularProbes, Inc. Eugene, Oreg.), and have the advantage of a large Stokesshift to minimize unwanted fluorescence excitation by the quenchingbeam. Furthermore they have a large stimulated emission cross section,as evidenced by their widespread use in dye lasers, and they also havebeen successfully used in two-photon excitation microscopy (Denk et al,Science 248:78 (1990)). The dye coumarin 1(7-diethylamine-4-methylcoumarin) can be excited by two photonexcitation by a Ti-sapphire laser set to about 700 nm. Quenching can beby the frequency doubled 950 nm output of the Ti-sapphire laser, toproduce pulses at 475 nm. The widely used dye, lucifer yellow, has theadvantage of a very large Stokes shift, and can be two-photon excited bythe output of a Ti-sapphire laser at 850 nm or single photon excited bythe frequency doubled 850 nm output (i.e., 425 nm), and the quenchingbeam can be the frequency doubled 1080 nm output of the Ti-sapphirelaser (i.e., at 540 nm). Because of Lucifer yellow's large Stokes shift,it is possible to quench by tuning the quenching laser in the optimum ofthe emission band, which is also at the long wavelength cutoff region ofthe frequency doubled Ti-sapphire laser. It may desirable to lengthenthe pulse width for the quenching beam to, say, 10 picoseconds, both toeliminate two-photon fluorescence excitation in the UV portion of theLucifer yellow excitation spectrum, and also to sharpen the spectralspread so that a narrow band rejection beam filter 27 can eliminateunwanted direct and scattered light from the quenching laser from addingnoise to the fluorescence signal recorded by the photodetector 23.Alternatively, or in addition to spectral filtering of this direct andscattered quenching laser light, the output of detector 23 can be gatedto be unresponsive during the time such direct scattered light from thequenching laser is falling on it. Yet another means to reduce quenchingbeam photons from reaching detector 23 is to replace dichroic beamsplitter 15 with a polarizing beam splitter which reflects planepolarized light from lasers 70 and 71 and transmits the opposite planeof polarization. To the extent that the fluorescent emission isdepolarized, it will be able to be partially transmitted through thepolarizing beam splitter. (In case the fluorescent emission ispolarized, a quarter wave plate between the polarizing beam splitter 15and the specimen will rotate the plane of polarization by 90 degrees, soit will pass through the beam splitter and reach the detector.)

These examples of fluorescent dyes have been discussed principallybecause they can be excited and quenched with the wavelengths availablefrom Ti-sapphire lasers, which unfortunately have a wavelength gap fromabout 540 nm to about 680 nm, which is in the region of excitation orquenching of some dyes which otherwise would be good candidates for thepresent invention. The use of optical parametric oscillators, or lasersable to operate within this wavelength gap of the Ti-sapphire laser,will permit the use of such dyes. A particularly promising class of dyefor use in the present invention are the inclusion compounds of thecyclodextrin molecule and various laser dyes occupying its hydrophobiccentral cavity. The extensive search for laser dyes has found dyes whichin many aspect are ideal for the present invention, having a highquantum efficiency, a high stimulated emission cross section and a lowground state absorption at the wavelength of stimulated emission. Theuse of a cyclodextrin host allows hydrophobic dyes which ordinarily arenot suitable for aqueous environments to operate in a hydrophobicmicroenvironment within an aqueous environment.

In the embodiments described so far, the mechanism focusing laser lighton annular aperture 21 insures that the light leaves this aperturegenerally coherently and in-phase. However such illumination is notoptimal for reducing the width of the central minimum. The reason isthat, starting from a point on the first minimum ring of an Airy discpattern, movement towards the central maximum or movement away from it(towards the first bright ring of the Airy disc) both lead to anincrease in intensity, however in the two directions the oscillatingelectric field vector of the light is opposite. The problem thisopposite electric field causes may be seen by considering just thecontribution of two small segments of aperture 21 on opposite sides fromthe center. Each of these segments projects its own Airy disc in thespecimen, positioned so that the first dark ring of both of these Airydiscs passes through the central minimum. However a small distance fromthe central minimum, the area between the central maximum of one of theAiry discs and the common central minimum coincides with the areabetween the central minimum and the first bright ring of the second Airydisc. Since the two light sources are coherent and in phase, these areaswill have opposite electrical vectors, and therefore there will bedestructive interference. The result of this cancellation is that thenet light intensity grows relatively slowly with distance from thecentral minimum. One solution to this problem is to employ as quenchinglaser 11 or 71, a laser with inherently low coherency. The excimer laserhas the right coherency properties, but unfortunately is a pulsed laserwith too low a frequency to be practical in a scanned laser device suchas the device of FIG. 1, however it might be usable in the more parallelembodiments of the invention.

FIG. 8 shows a solution to this problem an embodiment of the inventionwhere diametrically opposite sources of quenching light, which arepositioned so as to summate in the specimen to create the centralminimum, produce light which is 180 degrees out-of-phase. Therefore inthe areas near the central minimum, light from the two sources willconstructively interfere, causing the intensity to rise sharply withdistance from the central minimum, thereby decreasing the width of thecentral minimum. Measured in the plane of focus, the sharpening due tojust two out-of-phase sources is in just one dimension. Unfortunately totry to extend each source into a semicircle, so that together theyencompass the entire annular aperture, will still produce a centralminimum in which only one dimension is narrowed by the process. Howeverin arraying two or more pairs of out-of-phase sources at the corners ofa regular polygon, such that coherence between different pairs isminimized, will solve the problem by approximating a radiallysymmetrical central minimum, narrow in two orthogonal dimensions.

In the device of FIG. 8, the annular aperture 21 of FIG. 1 is replacedby a hexagon of illuminated optical fibers. Light from excitation laser10 is focused by lens 12 onto one end of optical fiber 91, and lightemerging from the other end 98 of fiber 91 is reflected from dichroicmirror 14, and directed through beam scanning means 16 to lens 18, whichfocuses this excitation light to a spot on the specimen 19. Three lasers11, 11′ and 11″, each within the band of effective quenching, but ofwavelengths far enough from each other that they are mutuallyincoherent, have their output beams focused by lenses 92, 92′ and 92″onto three pairs of phase-preserving optical fibers, one pair containingfibers 94 and 95 being illustrated along their full length. Thenon-illuminated ends of these fibers, for example end 96 and 97, are inthe plane 99, that is conjugate to the plane of focus of specimen 19,and consequently is the same plane occupied by annular aperture 21 inFIG. 1. By mechanically adjusting fiber end 96 with respect to fiber end97 (the means for such adjustment is not illustrated) the quenchinglight emerging from these ends on plane 99 is 180 degrees out-of-phase.As shown in FIG. 9, which shows a cross section through plane 99, theends 96 and 97 are at diametrically opposite vertices of a hexagon, andthe separation between ends 96 and 97 is such that at the wavelength oflaser 11, the Airy discs of these fiber ends projected into the specimenhave their first minima passing through the central point of the centralmaximum of the Airy disc projected by lens 18 on the specimen 19 fromlight emerging from the end 98 of fiber 91. Dichroic mirror 14 makes theend 98 of fiber 91, conducting excitation light, conjugate to thecentral point of the hexagon of no-nilluminated ends 96, 96′, 96″, 97,97′ and 97″, but it is also possible to locate the end 98 physicallywithin that hexagon, so that dichroic mirror 14 is unnecessary. (FIG. 8,it will be realized has described just the system for illuminating thespecimen, and the viewing system, using a dichroic mirror, is the sameas in other embodiments.)

Another way to provide the necessary incoherence between the variouspairs of out-of-phase sources, is for them to have the same wavelength,but to be on at different times, so that interference is impossiblebetween one pair and another pair. The device of FIG. 10, which is thepreferred microscope embodiment of the present invention, uses a pulsedlaser 71 directed by lens 92 into four phase-preserving optical fibers100, 101, 102 and 103 (six or any even number of fibers greater than twocould have been used instead of four), so that a single coherent pulsesimultaneously enters all four fibers. (Though these four fibers havebeen shown receiving the focused beam from laser 71 arranged in a line,more likely they would be arrayed in a compact square, or the laserwould be directed into just one fiber, the output of which would besplit twice by well known methods in the art of fiber optics.) Two ofthe fibers 100 and 101 are short, and have a difference in length of onehalf the wavelength of the laser light, in order to make their outputsout-of-phase. (Or they have the same length and the phase difference isadjusted by mechanically adjusting their ends,) The two remaining fibers102 and 103 (illustrated for just part of their lengths) are long (alsowith a small difference in length to insure that their output isout-of-phase), and their length is such that by the time the light pulseemerges from them, the light has stopped exiting the short fibers (thatis to say that the length difference between the short and the longfibers is equal or greater than the laser pulse duration times the speedof light). The exciting light from laser 70 emerges from the end 98 offiber 91, end 98 being in the center of the square formed by thenon-illuminated ends 104 to 107 of the fibers 100 to 103. A diagram ofthe ends of the fibers through the plane of these ends is shown in FIG.11.

As in the embodiment of the invention shown in FIG. 7 excitation laser70 of FIG. 10 emits a pulse first, followed by quenching laser 71. Thefluorescent emission from the specimen is reflected by dichroic mirror108 then passes through pinhole 22 onto a photodetector 23, the outputof which modulates either the spot of a display cathode ray tube, rasterscanned in synchrony with the scanning of the superimposed excitationand quenching beams across specimen 19, by scanning means 16, or scansthe write address in an image frame store. As in earlier embodiments,there is assumed to be a blocking filter (unillustrated) in front of thedetector 23, to eliminate both excitation and quenching photons fromentering the detector, and furthermore, the detector may be time gatedto be insensitive during the times the excitation and quenching lasersare on, and their light may be directly reflected back by the specimen.The convex lens 18 here symbolizes the complex imaging optics of acompound microscope, which may include in addition to an objective, aneyepiece lens and perhaps other lenses and other optical elements aswell.

FIG. 12 shows the form of the present invention adapted to the field ofphotolithography, which enjoys the benefits of quench sharpening, whileat the same time applying an image to the wafer with massiveparallelism, to achieve a high throughput. When the present invention isadapted to microlithography, the excitation laser excites a photoactivemolecule in a photoresist layer of a wafer to be made into microchips.The photoactive molecule then enters a transient excited state analogousto the excited state of a fluorescent molecule, which at least in manycases should be susceptible to optical quenching, though the period ofvulnerability of the state to quenching is far shorter than the case ofthe fluorescent molecule. When quenched the photoactive molecule revertsto its ground state. However if not quenched it has a probability ofcausing a lasting change, constituting the “latent image,” which willchange the vulnerability of the photoresist layer to some developmentprocess, where for example exposed areas become insoluble and remain onthe wafer, protecting it, while non-exposed areas can wash away, leavingparts of the wafer vulnerable to some etching process.

In the microlithographic embodiment, excitation pulsed laser 70 isfocused by lens 12 to a pinhole 13 and the beam emerging from pinhole 13is collimated by lens 115 and the emerging plane wave 116 is directed onmicrolens array 117, which projects focused spots from laser 70 on aprojection mask 118. The lenses of microlens array 117 are arranged in aregular hexagonal array, so that the spots projected onto mask 118 areat the centers of the hexagons of such an array. Where a spot ofexcitation light is imaged by any of the microlenses on a transparentregion of mask 118, the light emerging, for example ray cone 119, passesthrough dichroic beam splitter 14, and is focused by lens 18 to thephotoresist layer 120 on wafer 121. Because array 117 can simultaneouslyfocus thousands of spots, an image can be transferred from the mask 118to the photoresist layer 120 on wafer 121 with an enormous degree ofparallelism. Light from ultrafast pulsed quenching laser 71(synchronized with laser 70 as described above) is imaged by lens 122onto a bundle of 32 phase preserving optical fibers, arranged into fourbundles of eight fibers each, two bundles 123 and 124 of which areillustrated, each with only four out of the eight fibers, to simplifythe illustration. Each bundle has a different length, so that the pulsesfrom the laser emerge at different times from each of the bundles, andtherefore light emerging from fibers in one of the bundles cannotinterfere with light emerging from another bundle. The rectangle 131 inthe lower right corner of FIG. 13 shows the ends of each of the 32fibers in the plane 125 of FIG. 12. The fibers in bundle 123 areschematically shown in rectangle 131 as circles with either a lower case“a” or a capital “A,” those in the lower case “a” having light whichemerges 180 degrees out-of-phase with respect to light emerging from thecircles with capital “A.” (Note that the diameters of the circles inFIG. 13 are not drawn to scale, in relationship to the dimensions of thehexagons.) Similarly, fibers in bundle 124 terminate in the circles witha lower case “b” or a capital “B,” with a similar out-of-phaserelationship between the lower case “b” and the capital “B” fibers, andso on for the unillustrated fibers ending in the circles with c's andd's. Light emerging from the unilluminated ends of the fibersterminating in plane 125 (which has been drawn separated from the endsof the fibers for clarity, but is actually assumed to be in the sameplane with these ends), is focused to infinity by lens 126, and thelight passing through lens 126 is directed onto microlens array 127,which is a rectangular array with the same aspect ratio shown in FIG.13, and which is of the appropriate lens spacing to image the ends ofthe fibers at plane 125 to the repeating hexagonal pattern at plane 128shown in FIG. 12. (One typical hexagon is labeled 132 in FIG. 13.) Thescale of the pattern at plane 128 of the focused images of the fibers atplane 125 is such that when the light leaving plane 128 is reflectedfrom dichroic mirror 14, and imaged by lens 18 onto the photoresistlayer 120, there is an image of each spot from the plane of the mask118, where the mask is transparent, projected into the center of one ofthe hexagons of the array of fibers conducting the quenching radiation.It should be noted that each hexagon has the same coherency and phaserelationship as the device shown in FIGS. 8 and 9, namely quenchinglight imaged on opposite sides of the hexagon is coherent and 180degrees out-of-phase while light imaged on adjacent vertices is mutuallyincoherent, in this case because the imaging occurs at different timesdue to the different time delays in the different fiber bundles such asbundle 123 and 124. It is assumed that the pulses from the lasers 70 and71 are short enough, e.g., some hundreds of femtoseconds or less, thatit is possible to have four different delays for the fiber bundlesconducting the quenching light, so that quenching still arrives at thephotoresist layer 120 before the excited state induced in thephotoactive molecules by the light pulse from laser 70 has decayedeither to a non-quenchable triplet excited state, or has initiated thelatent image local chemical change. Therefore, provided sufficientaccuracy can be provided in the fabrication of the two microlens arrays117 and 127, and the relative phases can be maintained through thesuccessive imaging steps for the quenching radiation, the projection inthe photoresist of each hexagon provides satisfactory quench sharpeningfor the spot of excitation in its center.

The array of spots projected onto the photoresist layer 120 is convertedinto a continuous two dimensional image of the mask 118 by laterallytranslating the mask by motor means 129 and laterally translating thewafer 121 in opposite direction by motor means 130, with a ratio ofvelocities equal to the magnification of lens 18, so that duringscanning, the image of mask 118 maintains a stable relationship with thephotoresist layer.

As shown diagrammatically in FIG. 14, the direction of translation ofthe mask (shown by arrow 140) is such that it is at a small angle 141with respect to one of the major axes 142 of the hexagonal array ofexcitation spots projected by microlens array 117, so that thetrajectory of two adjacent spots in the array form two parallel linesseparated by less than the post-quenching resolution distance whenreferred to the photoresist layer, and such that by the time the entiremask has been scanned in front of the microlens array 117, thetrajectories of all the excitation spots form a continuous gratingpattern on the photoresist, with no gaps or regions of overlap. As isevident from FIG. 14, the angle 141 would be much smaller as the numberof hexagons per row is increased from the five in the figure, to perhapsa thousand or more, as imagined for the device of FIG. 12. It is alsopossible to have an angle 141 which is a small multiple of the anglerequired for no overlap, in order to have the same point on the resistexposed by different sets of excitation/quenching microlenscombinations, so that any defects in a particular microlens can becompensated by proper exposure from another microlens.

Unlike the apparatus shown in FIGS. 8 and 9, where only thecontributions from six points must be considered when computing theshape of the central minimum for the quenching beam on the photoresist,in an array such as in the embodiment of FIG. 12, contributions fromfarther points must also be considered. Fortunately, in the case of ahexagonal array, neither the six second nearest neighboring points, northe 12 third nearest neighboring points contribute substantially to theintensity at the central point of the central minima, because at theirparticular distances, they are near minima in the Airy discdistribution. Interestingly this is not the case for square arrays.However, even in square arrays, when diagonally opposite points areout-of-phase, then for each central minimum, the contribution from thehigher order points always cancels.

An additional advantage of creating the central minimum of quenchingradiation by superposition of out-of-phase sources, is that the twoopposite sources can be closer than when zero central intensity isproduced by over lap of first minima of in-phase or mutually incoherentcomponent sources. In theory, with out-of-phase sources, the centralpoint will have zero intensity for all distances between the oppositesources but when two close the peak power in the specimen is severelyreduced. An ideal spacing appears to be about half the spacing from eachother dictated by the requirement to overlap first minima of thecomponent Airy discs or in other words, the image in the specimen of thecentral maximum of one of the two out-of-phase sources coincides withthe first dark Airy disc ring of the other source. Compared to thespacing where the dark rings coincide, such reduced spacing yields aboutdoubling of effective resolution for a given exposure of the specimen toquenching beam power. Finally, because of the relatively tolerance ofresolution on intersource spacing, the sources can have a larger areathan with the requirement of superposition of first dark rings.

As the technology for making large matrices of shutters evolves, itwould probably be preferable to implement the technology of the paralleldevice of FIG. 12, by having the information about mask 118 encoded in adigital memory, which would be read out with massive parallelism to ashutter array positioned at the plane of the mask 118, so there would beno need to move the mask

It is therefore believed that the present invention, and in particularthe preferred microlithography embodiment shown in FIG. 12, may allowmicrochips of exceedingly fine critical dimensions to be fabricatedeconomically, for the most part, using tool and processing componentscurrently in place at chip fabricating facilities. Because these chipswould not have to be exposed to ionizing radiation in the exposure ofthe photoresist, annealing steps required in chip manufacturingprocesses requiring ionizing radiation could be eliminated, and theresulting absence of any defects which might escape the annealingprocess would lead to chips of greater reliability.

In the embodiments of the present invention shown in FIGS. 8, 10 and 12,the requirement that different pairs of out-of-phase sources notoptically interfere with other pairs was met 1) by making the pairs havedifferent wavelengths or 2) by having light emitted in pulses that didnot overlap in time. FIG. 15 shows how the device of FIG. 1 can bemodified to have the advantages of out-of-phase sources withoutrequiring separate lasers for different pairs, or requiring opticalfiber delay lines. Quenching light from pulsed laser 11 is focused bylens 140 onto an opaque filter 141 with 4 transparent openings 142, 143,144 and 145, containing optical delay plates, delaying the light by 0°,90°, 180° and 270°, respectively). FIG. 16 shows a magnified view of thefilter 141. The elements in FIG. 15 are assumed to be part of amodification of the device shown in FIG. 1, so that the quenching lightleaving filter 141 pass through dichroic beam splitter 14 and then viaelements 15, 16, 17, and 18 are directed to the specimen 19, as shown inFIG. 1. (As described earlier, far better performance is expected whenthe lasers are pulsed lasers, but to reduce confusion, the continuouswave lasers 10 and 11 of FIG. 1 have been shown in this example.)

With respect to interference, two sources of mutually coherent lightwith a 90° phase difference act substantially like mutually incoherentsources. Therefore the quenching light leaving openings 142 and 144 willact as if it is incoherent with respect to the light leaving openings143 and 145, and thus the image in the specimen of openings 142, 143,144 and 145 will be similar to the image in the specimen of fiber ends104, 107, 105 and 106 of the device of FIG. 10. Thus by replacing thetoroidal lens 22 and the annular aperture 21 of FIG. 1 with the lens 140and the filter 141 of FIG. 15, a substantial improvement in resolutionis gained.

The use of 90° and 270° phase shifts to produce non-interference caneasily be generalized to the devices of FIG. 12, where multiple pointsare simultaneously resolution enhanced. However, a square rather thanhexagonal matrix is required. FIG. 17 shows such a matrix, where thephase shifts within each of the circles is shown, and where the spaceoutside the circles is opaque. Such a matrix filter could be producedfor millions of simultaneously illuminated points, using the well knowntechniques for producing phase shifting masks for the microlithographicindustry. The matrix shown in FIG. 18 adds intermediate phase shifts at45°, 135°, 225° and 315° to make the shape of the resolution enhancedspot profile more closely approach a circle, when this is desired. Oneimmediate advantage of use of a matrix of illuminated points, is thateach projected spot of quenching radiation in the specimen services fourrather than one excitation spot, so in cases when the resolutionachievable is limited by the ability of the specimen to tolerate highpowers of quenching radiation, use of simultaneous multiple excitedpoints may result in improved resolution.

FIG. 19 shows a modification of the device of FIG. 12, using a phaseshifting matrix as shown in FIG. 17 or FIG. 18. This matrix filter 150is located at the plane which is conjugate to mask 118 with respect todichroic mirror 14. The microlens array 151 is a square rather than thehexagonal array 117 of FIG. 12, but works in the same way. Filter 150 isilluminated by pulsed quenching laser 71, the output beam of which isexpanded by lenses 152 and 153. Filter 150 is positioned, and the scaleof its openings are such, that the central minima of quenching lightemanating from filter 150, projected onto the photoresist layer 120,coincide with the central maxima of light passing through themicrolenses in array 117, which because they pass through a currentlytransparent part of mask 118, reach the photoresist layer 121. Thesynchronized translation of the photoresist layer 120 and the mask 118are as described for the device of FIG. 12. It will be appreciated thatthe device of FIG. 19 is simpler than the corresponding device of FIG.12, but there is a possible problem in case the required brevity of thepulses from laser 71 causes a spectral broadening.

When a phase retarding element has zero chromatic dispersion, then thetwo wavelengths will be retarded by the same time interval, not the samephase. This means that if the element produces a 180° phase shift forone of the wavelengths, in general the phase shift for the remainingcomponent will not be 108°. In devices such as that in FIG. 19 thosewavelengths which are not exactly out-of-phase will increase theintensity of light at the central minima of quenching light, reducingthe efficiency of the device. Optical fibers have been developed with ahigh negative chromatic dispersion, meaning that long wavelengths travelslightly slower through the fiber than shorter wavelengths. If fiber 100in the device of FIG. 10 were an appropriate length of such a fiber, andfiber 101 were a zero chromatic dispersion fiber, it should be possible,over a range of wavelengths, to make each wavelength emerging from fiberends 104 and 105 have a phase difference of 180°. Such a scheme could beapplied to the microlithographic embodiment of FIG. 12 more simply thanto that of FIG. 19.

FIG. 20 schematically shows an alternative solution to maintaining anout-of-phase relationship over a range of wavelengths. Fibers 100 a and101 a in FIG. 20 correspond to fibers 100 and 101 in FIG. 10, and othercorresponding elements have also been labeled with the suffix “a”.Instead of illuminating the fibers by the same focused laser as in FIG.10, a beamsplitter 160 creates a pair of beams from the laser output,one which is directly focused on the lit end of fiber 100 a and theother of which, after reflection by mirror 161, is passed through achromatic phase compensating assembly, consisting of prisms 162 and 143and a phase retarding filter 164 at the chromatically dispersed focus ofthe laser beam, between 162 and 163. Prism 163 reassembles the spectralcomponents into a single beam, which is then focused by lens 165 on thelit end of fiber 101 a. Because each spectral component is focused to aparticular spot on filter 163, by adjusting the thickness of the filterat that point particularly for the component, each component can beseparately adjusted to be exactly 180° out-of-phase at plane 99 a. Withthis arrangement, any dispersion within the fibers 100 a and 101 a canalso be compensated. It will be appreciated that there is probably noneed in this device for a literal filter 164, but by suitably tiltingthe apparatus, the phase shift can be changed as a function ofwavelength. It will also be appreciated that for the prism 163 toefficiently undisperse the light dispersed by prism 162 some focusingmeans should be included between the prisms. However such opticaldevices in which a beam of light is first dispersed and thenundispersed, are so widespread in the art, for example for pulsestretching and compression for ultrafast lasers, that this schematic inFIG. 20 will be sufficient. Needless to say, when a chromatic phasecompensating device, or negative dispersion optical fiber must be usedto provide a constant phase shift over a spectral region, the devicesuch as shown in FIG. 12 is more appropriate than that shown in FIG. 19.To enjoy the benefits of the simplicity of the device of FIG. 19 in thefield of microlithography, it might be necessary to develop photoresistswith unusually long lifetimes for the optically quenchable excitedstate, enabling quenching pulses to be long enough to approachmonochromaticity.

In microscopic applications of the present invention, on the other hand,the quenchable state fluorescent lifetime is already, usually, more thanhundreds of picoseconds, so the quenching pulse can be long enough toapproach monochromaticity. FIG. 21 illustrates a microscope which uses aphase shifting filter such as shown in FIG. 17 to achieve massiveparallelism in exposure, allowing rapid frame rates to be coupled withhigh pixel count images with extraordinary resolution. This microscopeis adapted from the device of FIG. 7, and elements with the same numbersas in the former figure are explained in the text for that figure.Elements to synchronize the lasers, shown in FIG. 7, have not beenrepeated in FIG. 20 for simplicity, as have other elements withobviously similar functions. Replacing the annular aperture 21 of FIG. 7is an elongated square matrix filter 160 similar to that shown in FIG.17, but with about 200 squares in the long dimension and 20 squares inthe short dimension. Matrix filter 160 is oriented in the FIG. 20 so itslong dimension is vertical. Matrix filter 160 is illuminated by pulsedlaser 71, the output beam of which is expanded by lenses 161 and 162(which may be cylindrical) to uniformly illuminate filter 160. Pinholearray 163 has pinholes in a square array, positioned so that the mirrorimage of the pinholes with respect to dichroic beamsplitter 14 coincidewith the centers of the squares of the matrix filter 160. Thisarrangement insures that the central maxima in the specimen 19 from thepinholes in array 163, as illuminated by the beam expanded excitationlaser 70, will coincide with the central minima produced by thequenching light emitted from matrix filter 160. Lenses 164 and 165perform the beam expansion for laser 70. Beam deflector 16, which hasonly to deflect the beam in one dimension so that the coincident imagesof the elongated filter 160 and array 163 moves in the direction oftheir short axis. More accurately the axis of movement is at a slightangle with respect to the short axis, so that as the scanning bydeflector 16 takes place, the coincident excitation maxima and quenchingminima traverse paths similar to that shown in FIG. 14. Each of theilluminated spots in the specimen 19 is imaged on a separate lightdetector in detector array 166, which ideally would have detectors suchas avalanche photodiodes which approach the sensitivity ofphotomultiplier tubes. For each position of deflector 16, each detectorin array 166 is receiving light from a unique position (x, y) in thespecimen. During scanning, the output of array 166 is directed to aframe store memory, along with a scan position signal from scan drivecircuit 25, so that the output for each detector in array 163 can bedirected to the appropriate location in the frame memory. The framememory is raster scanned continuously to produce an image of thespecimen on monitor 26. Because lateral scanning is required in just onedimension, a fast scan rate is possible, which should allow real-time3-D reconstruction of the specimen, by adding a depth scan. Although thedevice of FIG. 10 is preferred in terms of fitting into the mainstreamof point scanned microscopes, the embodiment shown in FIG. 21 may haveadvantages in many applications, where its imaging speed, resolution andpossibility of real time 3-D reconstruction may be useful.

For most microscopy and microlithography applications of the presentinvention, while it is desirable to restrict spot area as much aspossible in the lateral dimension, in the depth dimension, a high depthof field is preferred. For example in microlithography, combining anarrow lateral spot size with an elongated longetudinal spot, to make aneedle like exposed area, would allow the process to tolerate fieldcurvature of the objective, and would also allow the full depth of thephotoresist to participate in responding to the light. This would alsoinsure that the walls of the features etched in the resist have wallswith a profile perpendicular to the surface. In the case of microscopy,a needle shaped area of excitation would produce the equivalent of theconfocal microscope technique of summation of many sections at differentdepths, only the required specimen exposure would be just that for asingle section. Fortunately, the shape of the effectively exposed area,when quenching is produced by devices such as shown in FIGS. 8 or 10 hasthis very elongated shape. However there are also times when it isdesirable to restrict depth. In these cases, it is possible for thelateral resolution enhancement to be produced by one exposure ofquenching light, for example with the arrangement of FIGS. 8 or 10, butthen within the lifetime of the excited state, expose the specimen to anadditional pulse of quenching light produced by the interference of twoout-of-phase spots on the optical axis, one above and the other belowthe central maximum of excitation, thus boxing in the excited area inthe depth as well as the lateral dimensions.

The same resolution enhancement techniques described above formicrolithography are also applicable to the microfabrication of smallparts. Here the ability to manipulate a block of optically writeablematerial in three dimensions, would allow the production ofmicrominiature parts which may not be available by any other technique,as described in more detail in the parent application of thisapplication, U.S. patent application Ser. No. 08/275,967, now U.S. Pat.No. 5,886,911, and which is incorporated herein by reference. Similarlythe present technique could be invaluable in the optical storage ofinformation, both in the writing and the reading phases of such storage,again as described more fully in U.S. Pat. No. 5,886,911.

While in the embodiments of the present invention described in thisspecification, both excitation and quenching were carried out by focusedbeams of light, either or both of these roles might be implemented byfocused beams of other types of radiation, for example by X-rays, byfocused electron or other particle beams or by focused ultrasonicradiation. In the examples given, the radiationally quenchable excitedstates have been electronically excited states, however any other typesof excited state, including nuclear excited states, excited statesinvolving macroscopic quantum structures, molecular isomerizations, orcrystal lattice phenomena, for example, would also fall within the scopeof the present invention. In the examples given, focusing of theexciting and quenching radiation is provided by lenses, however otherdevices for focusing are known, including concave mirrors, tapered lightpipes and optical fibers, and these can be used for focusing in thepresent invention. The examples given have considered a specimen ortarget material with just one radiationally excitable species, howeverit is often useful in fluorescent microscopy of biological material toemploy two or more contrasting fluorescent stains, and the presentinvention could be used in such applications, for example by choosingtwo fluorophores which have the same excitation and quenchingwavelengths but differ in fluorescent lifetimes or emission spectra. Allthe examples given have employed scanning of a spot or line, but thepresent invention is also applicable to applications requiring selectiveillumination of just a single unscanned spot. In the examples given,just one point or line in a specimen is scanned at each moment, howeverit is possible to simultaneously scan multiple points, with the use ofNipkow discs for example, as in the microscope of Petran (U.S. Pat. No.3,517,980 (1970)) and in such devices, each scanned point isindividually subject to the resolution enhancement of the presentinvention. Thus the scope of the invention should be determined by theappended claims and their legal equivalents, rather than by the examplesgiven.

I claim:
 1. In apparatus for irradiating a selected region of a targetmaterial containing a radiationally excitable species in order to excitemembers of said species, including a source of exciting radiationadapted to excite said members and focusing means to focus saidradiation to a pattern having a central maximum at said selected region,a method of increasing the resolution of said apparatus including thesteps of: providing quenching radiation adapted to radiationally quenchthe excitation of said members; and applying said quenching radiation tosaid selected region to preferentially decrease the excitation in achosen part of said region, thereby increasing the resolution of saidapparatus.
 2. The method of claim 1 and including the additional step ofshaping the quenching radiation to a pattern with a central minimum. 3.The method of claim 2 and including the additional step of making thecentral minumum substantially coincident with the central maximum ofexciting radiation.
 4. The method of claim 2 including the additionalsteps of: providing at least two sources of coherent quenchingradiation; and focusing radiation from at least two of these sourcesonto the target material so the quenching radiation arriving at theselected region from one of these sources is out-of-phase with respectto radiation from the other source.
 5. The method of claim 4 andincluding at least one additional source of quenching radiation focusedonto the target material, and including an additional step insuring thatin the selected region radiation from the sources of quenching radiationof claim 4 and radiation from this additional source of radiationsubstantially do not interfere.
 6. The method of claim 5 wherein thestep insuring that that the radiation from this additional source ofquenching radiation substantially does not interefere with radiationfrom the quenching radiation sources of claim 4 includes the steps of:providing that the quenching radiation from the sources of claim 4 isemitted in synchronous pulses: and providing that radiation from saidadditional source is emitted in pulses which arrive at the targetmaterial at substantially different times than the pulses from thesources of claim
 4. 7. The method of claim 5 wherein the step insuringthat that the radiation from said additional source of quenchingradiation substantially does not interfere with radiation from thequenching radiation sources of claim 4 includes the step of focusingsaid additional source of radiation onto said selected region so that insaid selected region the phase difference between it and the quenchingradiation from said sources of claim 4 is in the class includingsubstantially 60°, 90°, 120°, 240°, 270° and 320°.
 8. The method ofclaim 2, wherein said step shaping said quenching radiation into apattern with a central minimum includes the step of passing saidquenching radiation through a plurality of optical fibers, each of saidfibers having an exit end where the quenching radiation exits the fiber,such that said exit ends are spaced from an optical axis.
 9. The methodof claim 1, including the additional step of passing said quenchingradiation through said focusing means.
 10. The method of claim 1,wherein at least one source of radiation emits light.
 11. The method ofclaim 1, including the additional steps of: delivering said excitingradiation in pulses; and delivering said quenching radiation in pulseswhich begin after the initiation of said pulses of exciting radiation.12. The method of claim 1 wherein said radiationally excitable speciesare in a class including: fluorescent molecules in a target material tobe examined; molecules in a target material consisting of a recordingmedium encoding information; and molecules in a photolithographicresist.
 13. The method of claim 1 wherein said apparatus is adapted tosimultaneously irradiate a plurality of non-adjacent regions in saidtarget material with exciting radiation, forming in each said region acentral maximum of exciting radiation, and for each of said irradiatedregions, to direct said quenching radiation so that a central minimum ofquenching radiation overlaps with the central maximum in the region,thereby simultaneously improving the resolution for each of theirradiated regions.
 14. The method of claim 1 and including anadditional steps of measuring radiation emitted by the irradiatedportion of said material and substantially preventing quenchingradiation from being included in the measurement of the radiationemitted by the irradiated portion of said material.
 15. The method ofclaim 14 wherein said measuring step uses a radiation detector andwherein said additional step for substantially preventing quenchingradiation from being included in the measurement includes the use of anoptical filter substantially opaque to said quenching radiation, saidfilter being located in the optical path between said specimen and saidradiation detector.
 16. The method of claim 14 including the additionalstep of delivering said exciting radiation in short pulses anddelivering said quenching radiation in short pulses of quenchingradiation which follow said pulses of exciting radiation, and whereinsaid additional step for substantially preventing quenching radiationfrom being included in the measurement includes a step of gating themeasurement off during said short pulses of quenching radiation.
 17. Themethod of claim 1, wherein said species has an excitation spectrum withat least one band where radiation of a wavelength within said bandproduces substantially no excitation of said species, and including thestep of substantially preventing the quenching radiation from excitingsaid members by the step of providing the quenching radiation of awavelength within said band.
 18. The method of claim 1 wherein saidexciting radiation excites members of said species by a two-photonprocess and wherein said quenching radiation quenches said members by aone-photon process.
 19. In apparatus for irradiating a selected regionof a target material containing a radiationally excitable species inorder to excite members of said species, including a source of excitingradiation adapted to excite said members and focusing means to focussaid radiation to a pattern in a focal plane at said material, saidpattern having a central maximum, means for increasing the effectiveresolution of said focusing means including: means for providingquenching radiation adapted to radiationally quench the excitation ofsaid members; means for shaping said quenching radiation into a patternwith a central minimum, whereby within the central minimum, theintensity of quenching radiation increases with distance from the centerof the central minimum; and means for overlapping said central minimumwith said central maximum, whereby, within said central minimum, anexcited member of said species is quenched by said quenching radiationwith a probability which increases with the distance of said member fromthe center of said central maximum.
 20. In apparatus for irradiating aselected region of a target material containing a radiationallyexcitable species in order to excite members of said species, includinga source of exciting radiation adapted to excite said members andfocusing means to focus said radiation to a pattern having a centralmaximum at said selected region, mean of increasing the resolution ofsaid apparatus including: means for providing quenching radiationadapted to radiationally quench the excitation of said members; andmeans for directing said quenching radiation to said selected region soas to preferentially decrease the excitation in a chosen part of saidregion, thereby increasing the resolution of said apparatus.